1. Field of the Invention
The present invention generally relates to a system and method for measuring the motor strength of a human thumb or finger.
2. Background of the Invention
Despite great technological advancements in the field of hand surgery over the past twenty-five years, grip and pinch strength are the only two ways to quantitatively measure motor strength in the hand. A grip meter can be used to assess these parameters to make assessments as to focal areas of strength and weakness in the hand that may be due to primary muscle pathology, nerve and motor end-plate disease, and/or disorders of bone and joint. Variations in pinch such as tip, chuck, or key pinch are commonly used to mimic various activities of daily living. The Medico-Legal Society and workers compensation communities, as part of disability and return to work assessments, extensively use hand surgery research, daily clinical practice and the data obtained from grip and strength measurements.
The limitation of grip and pinch tests I have determined lies in the fact that both of these testing modalities evaluate muscle groups that are innervated jointly by the median and ulnar nerves. Grip strength is a composite function with contributions from all the palmar digits, as well as synergistic firing of intrinsic and extrinsic digital flexors. The extrinsic flexors of the sublimis and profundus groups of the hand are dually innervated in the forearm, whereas the intrinsic flexors are primarily ulnarly innervated, and the radial two lumbricals are median innervated.
With pinch strength, the same issues exist. The abductor policies brevis is served almost entirely by the median nerve. The adductor pollicis and the first dorsal interosseous are nearly completely ulnarly innervated. The remaining muscles, the flexor pollicis brevis and the opponens pollicis, show wide variations in median and ulnar innervations.
A quantitative measure of forces generated in pure palmar thumb adduction and abduction, I have determined, could serve as an adjunct to grip and pinch strength in the following conditions:
osteo-arthritis pre-operation and post-operation;
rheumatoid arthritis pre-operation and post-operation;
thumb reconstruction after trauma;
reconstruction of congenital differences;
following tendon transfer surgery; and/or
following tumor resection and reconstruction.
Instruments that evaluate the function of the human hand can be divided into three general types. First, instruments that measure the motor strength of the thumb and fingers in various positions of pinch and grip. Second, sensory measuring devices that assess fine touch, sharp-dull, two-point discrimination, pressure and temperature sensation. Third, dexterity measuring devices that assess neuromuscular coordination such as pegboard or Moberg instruments.
While existing pinch strength devices assess global functioning of the intrinsic thumb muscles, there is no known instrument that significantly, substantially and/or completely isolates one or more muscles that are enervated by the motor branch of the median nerve. As the motor branch enters the thenar eminence, I have determined that it enervates the abductor pollicis brevis, the opponens pollicis, and the superficial head of the flexor pollicis brevis. Other muscles are powered by the terminal motor branches of the ulnar nerve. These muscles are the deep head of the flexor pollicis brevis, the adductor pollicis, the first dorsal interosseous, and some fibers of the opponens pollicis. The simple act of pinch is actually a complex interplay of all the above muscles, and therefore there is no way to independently assess the function of those muscles that are enervated by the median nerve alone.
Muscles that abduct, or bring the thumb out of the palm in a vector perpendicular to the palm, are innervated by the median nerve. Muscles that adduct, or bring the thumb back toward the palm, are innervated by the ulnar nerve. There is no known device that can measure the force generated by these muscles.
For example, one known apparatus for exercising the human hand is described in PCT Publication WO/018018A2, which is incorporated herein by reference. As shown in the schematic illustration of FIG. 1, the assembly 110 includes two major components, a tension member 112 and a compression member 114. The compression member 114 is formed of a resiliently compressible material, such as open cell polyurethane foam, and in the embodiment that is illustrated, is configured as a generally spherical ball 116.
The tension member 112 is configured as an elongate, unitary tether 118 formed of a suitable elastomeric material, such as flexible PVC or latex rubber, for example. The tether 118 includes a main cord 120 that extends through a bore 122 in the compressible ball 116. A thumb loop 124 is mounted on the lower end of the main cord so as to project from the bottom of the compressible member, and a stabilizing web 126 is formed on the upper end of the cord. The stabilizing web protrudes slightly above the upper end of the ball, and finger loops 130, 132, 134 and 136 are attached to the web by comparatively short, narrow elastic cords 140, 142, 144 and 146. The finger cords 140–146 extend from the stabilizing web at predetermined angles so as to develop the correct force vectors for property exercising the hand.
The thumb loop and the finger loops are sized to fit over the middle phalanges of their respective digits, and are also provided with outwardly projecting tab portions 148 which aid in placing the loops by providing a grip for the fingers of the opposite hand.
Tapered junctions 150, 152 are preferably formed where the web and the thumb loop join the main cord. In addition to providing added strength and resistance to tearing in these areas, the tapered junctions 150, 152 engage corresponding recesses 154a, 154b at the ends of the bore 122 through the compressible ball member, thereby providing a firmer, more stable fit between the two members.
Thus, when the exercise assembly is installed on a user's hand, the fingers and thumb are able to move through their full ranges of motion, with the tension member offering a predetermined degree of resistance in the extension-abduction or adduction direction and the compression member providing a predetermined degree of resistance in the flexion-adduction-opposition direction. Moreover, the resistance is apportioned properly amongst the fingers of the hand, and the confirmation of the assembly ensures that the muscles and tendons are exercised together in a coordinated fashion. In addition, the configuration of the compressible ball and the tether structure enable the assembly 110 to be used with either hand, by simply reversing the assembly and installing the cords on the corresponding fingers of the other hand.
Therefore, the apparatus shown in FIG. 1 in no way quantitatively measures the forces generated by the thumb in abduction and adduction.